Improvements in the picture received by television receivers is limited by two important factors which are not related to the receiver technology. The first limitation is the 525-line format made standard for the United States television industry many years ago (National Television Standards Committee or NTSC). This places a limit of 525 resolution lines from top to bottom. The second limit is imposed by the broadcasting bandwidth allotted to commercial television. The bandwidth for video signals in domestic television is limited by government regulation to approximately 4.2 mHz. and the method of transmitting/receiving television pictures limits the resolution to this bandwidth. Most present day television receivers do not use the entire 4.2 mHz bandwidth of the electromagnetic frequency but have a composite RF/IF/video bandwidth of approximately 3 mHz. This places a limit to available horizontal resolution. The resultant television picture is similar to something between super-8 and 16 mm movies but well below the quality available from 35 mm movies. The obvious way to improve television images is to increase the number of scan lines (Europe uses 625) which increases the bandwidth. Closed circuit television with 1000 scan lines and a bandwidth in the vicinity of 16 mHz has been demonstrated to give picture quality approaching 35 mm. film. Although some cable transmission is not limited by the bandwidth it is limited to the ability of conventional television receivers to only receive 525 lines. Although television signals are presently broadcast utilizing 525 lines of picture viewing, the actual number of lines seen by the viewer is some what less since some of the lines are used up for auxiliary transmission. A NTSC television signal consists of blanking pulses, luminance data, chrominance data and sync pulses.
A great deal of effort has been expended over the years to increase the fidelity of the picture received. If the picture does not have to be transmitted the problem is simplified since transmission bandwidth plays a lesser part in developing increased fidelity. Because of the crowded air ways there appears little likelihood that the bandwidth which can be used for television transmission will be increased although the increase use of cable may reduce this problem. The primary disadvantage of the various systems which have been developed is that they render existing receivers obsolete in that the systems are not compatible with existing receivers.
In standard television transmission the picture or scene seen by the camera lens is converted into electrons. The standard system of transmitting the picture or scene is to break up the maximum number of lines, i.e. 525 into two fields of 262.5 lines each. The sene is scanned twice and the resulting two fields generated are interlaced to form one frame. The scene is scanned left to right in a slightly declining direction and a signal is included in the transmission to stop the signal while the electron direction is returned to the left to begin another line slightly lower than the first scan. Each line is made up of a series of electrons or pixels which correspond in intensity to the shades in the scene for which the pixel represents. Black areas are present between each scan line because of the return to the left side of the picture.
One of the more recent suggestions for improving the fidelity of the viewing picture is to remove the black lines so the viewer is less conscious of scan lines. This is done by digitalizing the received signal so that one line is generated twice and displayed on the screen in a slight offset thereby filling in the normal black lines. Additional lines can be generated in the same manner. Although such a system does not actually increase the fidelity since the identical picture appears on the screen with the same pixels in parallel lines, to the viewer there appears to be greater fidelity.
U.S. Pat. No. 4,623,913 issued to Fling on Nov. 18, 1986, discloses a system where the conventional incoming signal is doubled in the receiver to produce twice the number of lines. U.S. Pat. No. 4,794,456 issued to Tsinberg on Dec. 27, 1988 relates to transmitting two different pictures and therefore using two broadcast bands. One transmits high definition pictures and the other the conventional picture. U.S. Pat. No. 3,200,195 issued to Davies et al on Aug. 10, 1965 allows transmission on one band but requires two viewing tubes or the equivalent, which are then combined into one picture by optical projection. IBM Technical Disclosure Bulletin, Vol. 21 No. 5, dated Oct., 1978 suggest the conversion of the 525 line, two frame signal to a four frame picture utilizing an analog buffer memory. Such a device is a charge-coupled device operated in analog mode and normally refreshed so the net gain of the refresher circuity just offsets the loss through the CCD shift registry. The buffer converts the two frames to four while holding the frames and then releasing them in sequence.